The present invention relates to a process-for producing ethylene polymers having excellent resistance to moisture vapor transmission when the resin is formed into a film. Further, the process of the present invention produces MVTR resins which have ease of processing into a film packaging product.
The process of the present invention preferably uses a catalyst comprising a cyclopentadienyl chromium compound supported on a solid refractory material, in slurry or gas phase reaction conditions.
Polymeric composition for uses such as food packaging and other applications where good barrier properties against moisture ingress have been known for many years. Use of HDPE (high density polyethylene) for this purpose is disclosed in Kirk-Othmer, 3rd Edition, page 489. However, many HDPE polymers are not readily processable, such as by extrusion, to form packaging products.
It is desirable to have an HDPE that has excellent moisture vapor transmission resistance (MVTR) and also is relatively easy to process into a packaging product, such as a film.
The general theory of permeation of a gas or liquid through a polymer film is that the permeation rate is the product of a diffusion term and a solubility constant of the gas-liquid in the polymer, each of which is often independent of the other. The process of permeation through a polymeric barrier involves four steps: absorption of the permeating species into the polymer wall; solubility in the polymer wall; diffusion through the wall along a concentration gradient; and desorption from the outer wall.
There are certain molecular structures that lead to good barrier properties in polymers. A practical problem, however, is that the property that might result in a good gas barrier very often also results in a poor water barrier. Polarity is a case in point. Highly polar polymers such as those containing many hydroxyl groups [poly(vinyl alcohol) or cellophane] are excellent gas barriers but also are among the poorest water barriers. In addition, they become poor gas barriers when plasticized by water. Conversely, very non-polar hydrocarbon polymers such as polyethylene have excellent water barrier properties and poor gas barrier properties. It is generally thought that in order to be a truly good barrier polymer, the material must have: some degree of polarity such as contributed by a nitrile, ester, chlorine, fluorine, or acrylic functional groups; high chain stiffness; inertness; close chain-to-chain packing by symmetry, order, crystallinity, or orientation; some bonding or attraction between chains; high glass transition temperature (Tg).
In some prior instances, co-extruded film including a polyolefin layer and polar resin layer such as EVOH have been modified to improve water barrier properties by including other substances, such as a wax. Thus, according to U.S. Pat. No. 5,141,801, the barrier properties of a co-extruded film having a crystalline polyolefin surface layer can be improved substantially by incorporating a wax into the polyolefin. The film has an interior polymer layer that prevents migration of the wax to the other surface of the film so that wax does not interfere with the printability or heat sealability of the other surface. The wax-containing polyolefin layer also protects the interior layer from adverse effects of moisture, such as when the interior layer comprises EVOH. An interior layer of EVOH may be sandwiched between wax-containing polyolefin layers to fully protect it from moisture.
Major processes for producing PE resins, including HDPE, include solution polymerization, slurry polymerization and gas phase polymerization. Catalysts for these processes include Ziegler-Natta catalysts, Cr catalyst (either in homogeneous form or on a solid support), and, more recently, homogeneous or supported metallocene catalysts. The metallocene catalysts can be either mono or bis cyclopentadienyl (Cp) ligands on a transition metal, such as Ti, Zr, Hf, Cr, etc. The Cp ligands in turn can be substituted by various groups.
CpCr catalysts are disclosed in U.S. Pat. Nos. 5,240,895; 5,302,674; 5,320,996; 5,393,720; 5,399,634; 5,418,200; and 5,593,931.
U.S. Pat. No. 5,418,200, for example, discloses ethylene polymerization using various CpCr +3 valence compounds on a refractory support, such as silica. The ""200 patent states that the polymers produced have a polydispersity or molecular weight distribution (MWD) greater than 10, and that the polymers have improved ease of processing, better melt behavior, and other desirable properties such as impact resistance and environmental stress crack resistance. Also, the ""200 patent points out that large blow molded products are superior when made with high MWD polymers, and that film is more puncture resistant when made from polymer with a high MWD.
The ""200 patent does not disclose MVTR properties for film made from the polymers produced per the ""200 process.
WO 96/19527 (PCT/US95/16570) discloses polyethylene films of advantageous (low) MVTR, wherein the polyethylene resin used to make the film is produced using a metallocene catalyst. The metallocene catalysts are not specifically described in WO 96/19527, but reference is made to co-pending application U.S. Ser. No. 08/093,501 for disclosure of the metallocene catalysts useful in the ""527 patent application.
According to the ""527 patent application, the polyethylene resin has a density in the range of from about 0.935 to about 0.965 g/cm3, a Mw/Mn less than about 3, and an article made using the resin has a water vapor transmission rate less than 0.54 gxc2x7mil/100 in2/day (0.183 g/mm/m2/day), preferably less than 0.4 gxc2x7mil/100 m2 day (0.135 g/mm/m2/day).
Thus, the films made from the polyethylene according to the ""527 patent application have a MWD or polydispersity below 3.
U.S. Pat. No. 5,183,792 is directed to producing polyolefin resin using a catalyst comprising chromium and titanium supported on silica. The polymer produced has a high melt index and a narrow molecular weight distribution (MWD). The narrow MWD is indicated in U.S. Pat. No. 5,183,792 as helpful in achieving a low MVTR. As stated in the ""792 patent at column 4, lines 35-40: xe2x80x9cThe product will have a high melt index (MI) and a low high load melt index/melt index ratio (HLMI/MI) and, as is observed when these two properties are high and low respectively, a low water vapor transmission.xe2x80x9d
Also, in Plastics Technology, August 1999, in an article by J. Krohn et al. titled xe2x80x9cKeep It Dry, Optimize Moisture Barrier in PE Filmsxe2x80x9d, at pages 60-61, the authors state xe2x80x9cThus, structure 3 excelled in barrier because it was the only one to have a skin layer of higher MI resin with narrower MWD, both of which contribute inherently to better barrier.xe2x80x9d
According to the present invention, a process is provided for making an ethylene homopolymer having a polydispersity above 4, and wherein the homopolymer is suitable for producing a film having a high barrier to transmission of water or gas, or both, which process comprises contacting ethylene with a catalyst comprising a cyclopentadienyl chromium hydrocarbyl compound on a solid support, under slurry or gas phase polymerization conditions.
The process of the present invention is especially advantageous in producing resins which have an MVTR less than 0.4, preferably less than 0.3, still more preferably below 0.25 grams of water per 100 square inches of film per day, for a 1 mil (one thousandth of an inch) thick film.
Preferably, the resins produced in accordance with the present invention have a polydispersity or MWD above 4, more preferably between 4.5 and 12, and most preferably between 4.7 and 7.5.
Preferred catalysts for use in the process of the present invention are mono or bis cyclopentadienyl chromium compounds, more preferably a mono cyclopentadienyl, on a solid support. Preferably, mono cyclopentadienyl contains one or more substituents. Preferred substituents are hydrocarbyl groups; particularly preferred is cyclopentadienyl substituted with five methyl groups. Preferably the solid support is silica. An especially preferred catalyst for use in the process of the present invention is pentamethyl CpCr(CH3)2Pyr on a solid support, where Cp represents cyclopentadienyl, and Pyr represents pyridine. Preferred co-catalysts are alumoxanes, especially isobutyl alumoxane (IBAO). Preferred catalysts for use in the present invention are described in more detail below.
We have found that use of catalysts such as the aforementioned especially preferred catalyst, under process conditions as described herein, produces substantially. linear polyethylene of density at least 0.955 g/cc, with nearly all of the terminal groups of the polyethylene polymer saturated. Preferably, the process conditions are selected to allow the catalyst to react under mechanisms that produce predominately saturated end groups, that is, the methyl end group to olefin end group ratio relatively high, at least 10, and preferably greater than 15, as measured by FTIR (Fortier Transform Infrared spectroscopy).
Preferred process conditions, particularly when using the aforementioned especially preferred catalyst, include polymerization reaction zone temperature between 50xc2x0 C. to 100xc2x0 C., more preferably 70xc2x0 C. to 90xc2x0 C., with a slurry ethylene concentration of 8-15 weight percent. Hydrogen is a preferred chain transfer agent.
Other particularly preferred process conditions are related to the extrusion and pelletization process, and the film process.
Ideal compound conditions, or pelletization process are those that do not alter the chemical structure of the polymer; preferably cross-linking or long chain branching are kept to minimum.
Among other factors, the present invention is based on our finding that unexpectedly low MVTRs can be achieved from relatively high MWD resins, greater than 4 MWD, wherein the resin is produced from CpCr catalysts as described in more detail below. The resins produced by the process of the present invention have excellent processability while still achieving excellent MVTR properties.